1. Field of the Invention
The present invention relates to a selective-calling radio receiver such as a pager and more particularly, to a selective-calling radio receiver equipped with an exchangeable battery such as a dry battery as its power supply.
2. Description of the Prior Art
A conventional selective-calling radio receiver or a pager has a voltage monitoring circuit for monitoring the voltage of a primary battery incorporated therein as its power supply in order to avoid malfunction due to voltage reduction of the battery. The monitoring circuit causes a Central Processing Unit (CPU) of the receiver to generate an alarm signal for exchanging the battery when the battery voltage thus monitored becomes lower than a given reference voltage. The user or holder exchanges the battery in response to the alarm signal.
FIG. 1 shows a time chart for explaining the operation of the receiver. It is seen from FIG. 1 that the voltage of the battery reduces as the times go by and a Low-Voltage Alarm (LVA) is produced when the voltage becomes to a first reference voltage V.sub.1.
As shown in FIG. 1, usually, the first reference voltage V.sub.1 is set to be a little higher than a second reference voltage V.sub.2 which is called as a "CPU stop voltage" here. The CPU becomes to stop or malfunction if the voltage of the battery lowers to the CPU stop voltage V.sub.2.
The first reference voltage V.sub.1 is higher than the second reference voltage V.sub.2, so that the CPU does not stop immediately even if the voltage of the battery lowers to the first reference voltage V.sub.1. As a result, the holder of the receiver has a sufficient time for exchanging the battery after the LVA alarm is produced.
Also, in the case that the conventional receiver is provided with an alarm means for producing the LVA alarm such as a vibrator through which a large current flows, the alarm means start to operate when the battery voltage becomes to the first reference voltage V.sub.1, so that the CPU is difficult to stop even if the battery voltage lowers than the first reference voltage V.sub.1 momentarily due to operation of the alarm means.
In this case, the CPU sometimes stops to operate because the battery voltage momentarily drops to the second reference voltage V.sub.2 and restarts to operate. The data stored in the Random Access Memory (RAM) of the receiver is initialized together with the restart. Therefore, there arises a problem that peculiar user data stored in the RAM disappears and the user must reinput his data into the RAM, which requires the user for some troublesomeness.
In addition, as described above, with the conventional selective-calling radio receiver, the alarm starts to produce when the battery voltage lowers to the first reference voltage V.sub.1 which is higher than the second reference voltage or the CPU stop voltage V.sub.3. Therefore, the holder or user is obliged to exchange the battery whose capacity is not exhausted completely in response to the alarm.
As a result, there is another problem that the remaining capacity of the battery is not used effectively, which means that the remaining capacity is wasted.